1. Field of the Invention
The present invention relates to a process for producing a coated film continuously, by which a coating material having a low viscosity can be continuously applied onto the surface of a substrate film at high speed to produce a coating thin layer to a uniform thickness.
2. Description of the Related Art
Generally, processes for producing a coating layer onto the surface of a substrate film are categorized into two main types: a roller coating process and a spray coating process. In the roller coating process, a coating material is applied through a roller onto the surface of a running substrate film, while in the spray coating process, a coating material is sprayed from spray nozzles onto the surface of a substrate film. The roller coating process is further categorized into many types such as a doctor blade coating process using a doctor blade to control the thickness of a coating layer, an air knife coating process using an air knife to control the thickness of a coating layer, a direct gravure coating process using a sculptured gravure roller, a reverse roller coating process using a plurality of rollers which rotate inversely, a dip coating process in which a substrate film is dipped into a coating solution bath, a kiss coating process, a squeeze coating process and the like.
In the roller coating process, a coating material having a high viscosity is usually applied onto the surface of a substrate film directly. On the other hand, a coating material having a low viscosity is usually accommodated in a pan so as to be applied onto the surface of a substrate film through an applicator roller, as is shown in FIG. 3.
In the process of applying a coating material accommodated in the pan, however, there are some problems as below described. First, it is impossible to use up all the coating material in the pan, and therefore, this process is not economical in the application of expensive coating material even though using a gravure roller with transfer rate of 90%. In addition, in the application of the emulsion type coating material, such a material has too low viscosity to be conveyed through the applicator roller to the point where the coating material is applied to the substrate film. To solve this problem, there is a way in which a viscosity adjustor (a leveling agent) is added to a coating material to increase the viscosity thereof to the level capable of being applied onto the substrate film. This way is effective as far as the leveling agent does not adversely affect the physical properties of the resultant coating layer; however, the leveling agent is not suitable for the production of a tight coating layer with gas barrier properties. Moreover, in the conventional coating process, the maximum speed of accurate coating is 200 m/min at most, and therefore, there is a demand for a process of applying a coating material at higher speed.
When an organometallic compound-system coating composition is used for producing a coating layer with gas barrier properties, there also arise problems as below described. The organometallic compound-system coating composition undergoes hydrolytic condensation by absorbing moisture present in the air. Therefore, If the roller coating process is employed, the coating material is always exposed to the air, resulting in problems such as an increase in the viscosity, change in the solid concentration, and gelation during the coating process. On the other hand, the spray coating process adopting a closed-type system is useful as far as there is no need to produce a tight coating layer such as adhesives; however, this process is not adequate for the production of coating layer having gas barrier properties, because the resultant coating layer lacks uniformity and continuity. The problem due to the exposure to the air also arises in a coating composition easy to be oxidized and photocurable coating composition.